The invention relates firstly to a process for improving, in a defined wavelength band, the spectral response of a photoconductive structure exposed to a luminous radiation.
In addition, the subject of the invention is an improved photoreceptive multilayer structure and a solar cell made of "single " thin films or of the type referred to as "tandem" or "multijunction".
It is perfectly well known today that photoconductive structures, such as solar cells, which are intended to receive luminous radiations are constructed around at least one photoelectric conversion layer consisting of a semiconductor material which absorbs the incident light photons when the electrical energy of these photons is higher than the optical absorption threshold of the semiconductor in question.
It will be recalled, here and now, that the optical absorption threshold, also called "optical gap" in the case of a semiconductor, corresponds to a photon energy threshold. In the case of a monochromatic light whose photon energy is lower than this threshold, the material is transparent; on the other hand, in the case of an energy higher than the threshold, the material is absorbent. In practice, and to the extent that the transition is not strictly sharp and that the concept of transparency depends on the thickness of the material, we shall define this threshold at an optical absorption of the order of 60% in the case of a semiconductor layer whose thickness is typical of those employed in thinlayer, that is approximately 1-micron, photoelectric structures.
The absorption threshold may be defined by the wavelength (in vacuum) of the light of same energy. Since energies and wavelengths are inversely proportional, the wavelengths which are shorter (bluer) than the threshold will be absorbed, whereas longer (redder) wavelengths will pass through most of the structure without absorption, and therefore without triggering the photoelectric processes which activate this structure.
It will also be noted that in what follows the name "spectral band" of a photoelectric structure will be given to the wavelength region in the case of which the structure actually converts into electrical current the light it receives.
As to the expression "charge diffusion length", this will define the average distance travelled by the charges (free carriers) within the semiconductor in question before these charges disappear by recombination (between the electrons and the holes).
The typical example which is to follow will allow these various concepts to be clarified. The case chosen refers to a solar cell containing amorphous silicon (aSi:H) of p-i-n type; that is to say whose photoelectric conversion layer comprises an intrinsic (i type) semiconductive sublayer inserted between a sublayer of p type and a sublayer of n type.
With this type of cell the optical absorption threshold is limited towards the red light by the optical absorption threshold of amorphous silicon.
While the wavelengths corresponding to a blue light are absorbed after a few tens of nanometres (1 nm =10.sup.-9 m) in the amorphous silicon layer, the wavelengths corresponding to a green light are absorbed only after a path length of a few hundred nanometres. In contrast, a "deep" red light (wavelength of the order of 700 nm) is absorbed only after having travelled a few microns in the material.
In order to broaden the spectral band and to push back towards the red the operating limit of photoelectric structures containing a semiconductor (solar cells, photodetectors, etc.), it has already been proposed to increase the thickness of these structures up to the technically acceptable threshold (a few microns).
It was quickly noted, however, that problems then appeared which were due to the relative imperfection of the semiconductors employed (in particular aSi:H), in terms of electron-hole electrical charges: if the photoelectric conversion layer is thick, the electron-hole pairs are intrained by a relatively weak electrical field, this being over a relatively long distance, and these pairs remain for a relatively long time in the semiconductor, where their lifetime is of the order of the duration of the charge recombination processes within the material. In this case, the electrical charges are therefore lost before being collected.
Insofar as the wavelength ranges corresponding to red light are more particularly concerned, it is now considered that a sublayer of i (intrinsic) type from 0.3 to 0.5 microns in thickness is sufficiently thick to collect a considerable part of the red light photons in the visible spectrum, while remaining sufficiently thin for the collection efficiency and the fill factor to remain acceptable.
It will be noted that, in order to obtain a photovoltaic yield of more than 10% in sunlight, various technical improvements have been proposed, which tend to circumvent the problem of optical absorption.
In particular, provision has been made for producing structures with a contact, or rear electrode, with a reflective surface which has a very high reflectivity, in order to at least double the optical path (for example in red light). Provision has also been made for roughening the mutual contact surfaces of the layers so as to obtain a diffusion of light and to produce its optical trapping within the structure in the wavelength range in question.
It will be noted that, in the case of solar cells, increasing the spectral response in red light is of major importance for the photovoltaic yield, since the sunlight to which these cells are exposed contains a very large proportion of "red" photons.
In particular, in order to extend the spectral band towards the red, a solution has particularly been proposed, consisting in integrating or incorporating into the semiconductor employed (based, for example, on amorphous silicon), a complementary semiconductor such as a silicon-germanium alloy which has an absorption threshold and a charge diffusion length which are lower than those of the first semiconductor referred to.
While in principle this solution appears highly attractive (the spectral band is, in fact, extended towards the red), the overall balance has, however, turned out to be negative, since the addition of the complementary semiconductor entails a lowering of the overall photoelectric current which can be supplied by the structure.